Last year I explained that I didn't think believing that Santa is real makes the Santa story any more fun, even for kids (see: I Believe in Santa Claus). So I decided I wouldn't tell my kids Santa is real.
How is this working out? You may be wondering...
Not quite as expected. Last year I asked Nicolas a few times whether he thinks Santa Claus is a real person or just a story. As I recall, I even stated directly a few times that Santa Clause is just a fun story we like to tell -- not a real person. Nico responded by insisting that Santa is indeed real. Of course last year I wasn't really sure whether Nico understood the difference between a real person and a fictional character.
Fast forward to this year. Nico spontaneously told me one day "Some people think Santa Claus is just a dream, but he's a real person." (I think he was influenced by the special The Year without a Santa Claus.)
I responded by asking him "Are you sure he's real and not just a story? In that Christmas movie he looks like a doll."
Nico thought about it, and said "No, he's real -- we saw him. You saw him too. At the place where we go see Santa." [At the mall or something, I assume...?]
Of course I could argue this point, but I figured I might as well let him think about it on his own for the time being. So I just said, "Oh, okay."
So the magic of Christmas defeats the atheist mom.... For this round!! ;^)